1. Field of the Invention
The present invention relates to a semiconductor laser and a method for fabricating the same. More particularly, the invention relates to the construction of a ridge-type semiconductor laser formed with II-VI compound semiconductor materials, capable of emitting blue light in a wavelength band from 400 to 500 nm, and a method for fabricating the same.
2. Description of the Related Art
As the materials with which semiconductor lasers are fabricated, III-V compound semiconductor materials, such as GaAs, have been generally used. Meanwhile, the recent rapid progress in the semiconductor crystal growing techniques have made it possible to grow p-type crystals of ZnSe group materials which are II-VI compound semiconductor materials, which had been believed to be impossible. As a result, the formation of PN junctions in the ZnSe group materials and the consequent fabrication of light-emitting diodes and semiconductor lasers using those materials have been reported. An advantage of the ZnSe group materials is their capability to emit blue light in a wavelength band around 500 nm.
FIG. 7 illustrates an example of a sectional structure of a conventional semiconductor laser 700 fabricated by using the II-VI compound semiconductor materials (M. A. Haase etal., "Blue-green laser diodes", Appl. Phys. Lett., Vol. 59, No. 11, pp. 1272-1274 (9 Sep. 1991)).
In the semiconductor laser 700, an n-GaAs buffer layer 72, an n.sup.+ -ZnSe layer 73, an n-ZnSSe layer 74, an n-ZnSe layer 75, an undoped CdZnSe active layer 76, a p-ZnSe layer 77, a p-ZnSSe layer 78, and a p.sup.+ -ZnSe cap layer 79 are successively laminated in this order on an n-type GaAs substrate 71 by a Molecular Beam Epitaxy method (referred to as MBE method hereafter). A polyimide layer 80, having grooves formed in stripes, is formed and functions as a current blocking layer on the p.sup.+ -ZnSe cap layer 79. Further, an Au electrode 81 is formed on the top surface of the laminated multilayer structure, and an In electrode 82 is formed on the bottom surface of the n-GaAs substrate 71. The CdZnSe layer 76 used as the active layer is about 10 nm thick. The above constitution is categorized as an oxide-film striped structure.
As described previously, the Zn--Se alloys which are II-VI compound semiconductor materials are capable of emitting blue light. However, the II-VI compound semiconductor materials have properties quite different from those of the III-V compound semiconductor materials. Consequently, some of the structures and fabricating methods which have been applied to the semiconductor lasers based on the III-V compound semiconductor materials cannot be allied to semiconductor lasers based on the II-VI compound semiconductor materials.
To oscillate a semiconductor laser efficiently with a low threshold current, it is required to confine both current (injected carriers) and light simultaneously so as to obtain as high a gain as possible in a light-emitting region. From this consideration, the oxide-film striped structure shown in FIG. 7 is not sufficient in confining both current and light, resulting in insufficient laser oscillation.
The most typical structure to effectively confine current and light is the BM structure (Buried Heterostructure), in which an active layer acting as a light emitting region is embedded in other layers. The BH structure is commonly used in the semiconductor lasers based on III-V compound semiconductor materials.
For the III-V compound semiconductor lasers, the ridge type structure has also been proposed. This is made by forming a ridge in the cladding layer and filling grooves which define both sides of the ridge with an insulator layer (a current blocking layer). While the refractive index of the cladding layer is higher than 3, that of the insulator layer is below 3, for the wavelength emitted by the III-V compound semiconductor laser. This difference in the refractive index between the cladding layer end the insulator layer is utilized to confine light within a specified region.
Efficient confinement of current and light can be achieved by the use of the BH structure or the ridge type structure. However, such structures which are well applicable to the III-V compound semiconductor lasers are difficult to fabricate with the II-VI compound semiconductor materials for the reasons of the properties as described below.
First, unlike the II1-V compound semiconductor materials, no II-VI compound semiconductor material allows itself to readily obtain a lattice matching, and therefore it is very difficult to form the BH structure from a combination of single crystal semiconductor materials in this group.
Secondly, the II-VI compound semiconductor materials are likely to suffer thermal deterioration when heated after epitaxial growth. On the other hand, the formation of the BH structure or the ridge type structure requires forming a mesa or a ridge after forming the DH structure, and to carry out the burying process by using another material. Consequently, the II-VI compound semiconductor materials in the DH structure has to be re-heated up to a crystal growth temperature or more. As a result, forming the BH structure or the ridge type structure with the II-VI compound semiconductor materials causes significant deterioration in the characteristics of the semiconductor materials.
Thirdly, a carrier concentration in the p-type ZnSe is around 1.times.10.sup.18 cm.sup.-3. This value is not sufficient for the cap layer to achieve a good ohmic contact. Therefore, contact must be done with as large a width as possible in order to reduce contact resistance and decrease the operating voltage. However, increasing the width of the mesa or the ridge in the BH structure or the ridge type structure for the purpose described above tends to bring a multiple mode laser oscillation. This makes it difficult to stabilize the laser oscillation in the basic lateral mode and increases the threshold current level.
Thus, it is very difficult to apply conventional design and fabrication processes used for the III-V compound semiconductor laser to the II-VI compound semiconductor laser.